Three-Phase Induction Motors rule the industrial and commercial sector due to their simple design, robustness and long-history of reliable operation. Once operational, the ease through which their speed can be controlled is commendable, however, plant technicians can face some challenges when it comes to starting the motor, which have the potential to adversely effect their lifespan.
During direction line starting and stopping, LV/MV motors can undergo line currents that are eight times that of nominal current; this is followed by rapid acceleration rate or torque that puts mechanical stress on the shaft. As a side effect, voltage dips are experienced in the adjacent electric network which can affect sensitive digital equipment.
In order to deal with these long-standing issues, regulatory agencies throughout the world have stepped up efforts by raising the motors’ design requirements. Various EU-regulations are already aimed at reducing CO2 emissions as well as promoting energy efficiency, particularly IEC directive 60034-30:2014. Motors have been classified according to their design efficiency, namely:
- IE1 (standard)
- IE2 (high)
- IE3 (premium)
- IE4 (super-premium)
In addition to dealing with longevity issues and environmental concerns, the increased efficiency demand has also been fueled by the industrial sector that aims to raise its competitiveness.
Several starting techniques are available that enable motors to be started and stopped in a technically sound and efficient manner. Star-delta-starter configuration is popular throughout the industry due to its ability to reduce inrush current as well as low space requirements.
But there are disadvantages to this approach as well. The technique sets the starting characteristics in stone, offering no adjustability, and making soft-stop impossible. The cable costs can also become a significant factor as the motor requires six connections. This makes the method suitable for small-scale systems, but for plant managers who want to raise their efficiency in the long-run, further investments are required.
The Auto-transformer offers the prospect of reduced inrush current but has higher space requirements along with being more expensive. It also offers limited number of starts per hour that makes it incompatible with highly demanding applications. The popularity of auto-transformers is declining as applications require greater flexibility with the passage of time.
Frequency converters are a popular choice as they are continuously adjustable, with companies using it for their drives. There are strong arguments in its favor, particularly its ability to keep the nominal torque, as well as its ability to maintain precise speed limitations. This leads to an improvement of both mechanical performance as well as electrical efficiency of the drive train.
However, frequency converters are more expensive as they require a control system. To protect the converters from storage damage, enclosures are often required in the proximity of motors, that further add costs. The converter is also capable of producing considerable amount of electromagnetic interference on the distribution network that can affect digital components. In order to counter these issues, investments need to be made, e.g. LCL filters, making the Frequency Converter option suitable for highly-demanding motor applications.
A more economical option compared to Frequency Converters is the Soft Starter. Featuring simple construction, the technology is sturdy and doesn’t require frequent maintenance.
The starting phase of the Soft Starter lasts for 30 seconds at the maximum, after which it switches to a bypass mode and allows current to flow to the motor. Heat dissipation is primarily used to limit the inrush current, but due to the short duration of its operation, power losses are insignificant. In fact, compared to the frequency converter, a Soft Starter has an efficiency of 99%. The setup is also simple, and technicians can be easily trained to operate it.
Typical applications for soft starters include starting/stopping of pumps, milling machines, blowers, mixers, agitators, etc. At the same time, it must be noted that Soft Starters aren’t fit for applications that require speed control. The latter is embedded within frequency converters but doesn’t hold true for Soft Starters as they can only limit the flow of current. Therefore, for applications that don’t require speed control, a Soft Starter is highly suitable; otherwise plant managers would have to foot the bill for a more expensive Frequency Converter system for their three-phase inductor motors.